1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a method and apparatus for communicating information in wireless communications systems including, for example, a cellular radio system.
2. History of the Prior Art
Cellular Telephone Service
Cellular mobile telephony is one of the fastest growing segments in the worldwide telecommunications market. Between 1984 and 1992, for example, the number of mobile telephone subscribers in the United States grew from around 25,000 to over 10 million. It is estimated that the number of subscribers will rise to nearly 22 million by year end 1995 and to 90 million by the year 2000.
Cellular telephone service operates much like the fixed, wireline telephone service in homes and offices, except that radio frequencies rather than telephone wires are used to connect telephone calls to and from the mobile subscribers. Each mobile subscriber is assigned a private (10 digit) directory telephone number and is billed based on the amount of "airtime" he or she spends talking on the cellular telephone each month. Many of the service features available to landline telephone users, e.g., call waiting, call forwarding, three-way calling, etc., are also generally available to mobile subscribers.
In the United States, cellular licenses are awarded by the Federal Communications Commission (FCC) pursuant to a licensing scheme which divides the country into geographic service markets defined according to the 1980 Census. Only two cellular licenses are awarded for each market. The two cellular systems in each market are commonly referred to as the "A" system and "B" system, respectively. Each of the two systems is allocated a different frequency block in the 800 MHz band (called the A-band and B-band, respectively). To date, the FCC has released a total of 50 Mhz for cellular services (25 MHz per system).
Mobile subscribers have the freedom to subscribe to service from either the A-system or the B-system operator (or both). The local system from which service is subscribed is called the "home" system. When travelling ("roaming") outside the home system, a mobile subscriber may be able to obtain service in a distant system if there is a roaming agreement between the operators of the home and "visited" systems.
The Cellular System
In a typical cellular radio system, a geographical area, e.g., a metropolitan area, is divided into several smaller, contiguous radio coverage areas called "cells." The cells are served by a series of fixed radio stations called "base stations." The base stations are connected to and controlled by a mobile services switching center (MSC). The MSC, in turn, connected to the landline (wireline) public switched telephone network (PSTN). The telephone users (mobile subscribers) in the cellular radio system are provided with portable (hand-held), transportable (hand-carried) or mobile (car-mounted) telephone units (mobile stations) which communicate voice and/or data with the MSC through a nearby base station. The MSC switches calls between and among wireline and mobile subscribers, controls signalling to the mobile stations, compiles billing statistics, and provides for the operation, maintenance and testing of the system.
FIG. 1 illustrates the architecture of a conventional cellular radio system built according to the Advanced Mobile Phone Service (AMPS) standard. In FIG. 1, an arbitrary geographic area may be seen divided into a plurality of contiguous radio coverage areas, or cells, C1-C10. While the system of FIG. 1 is, for illustration purposes, shown to include only ten cells, the number of cells may be much larger in practice. Associated with and located in each of the cells C1-C10 is a base station designated as a corresponding one of a plurality of base stations B1-B10. Each of the base stations B1-B10 includes a plurality of channel units, each comprising a transmitter, a receiver and a controller, as is well known in the art.
In FIG. 1, the base stations B1-B10 are located at the center of the cells C1-C10, respectively, and are equipped with omni-directional antennas transmitting equally in all directions. In this case, all the channel units in each of the base stations B1-B10 are connected to one antenna. However, in other configurations of the cellular radio system, the base stations B1-B10 may be located near the periphery, or otherwise away from the centers of the cells C1-C10 and may illuminate the cells C1-C10 with radio signals directionally. For example, the base station may be equipped with three directional antennas, each one covering a 120 degrees sector cell as shown in FIG. 2. In this case, some channel units will be connected to one antenna covering one sector cell, other channel units will be connected to another antenna covering another sector cell, and the remaining channel units will be connected to the remaining antenna covering the remaining sector cell. In FIG. 2, therefore, the base station serves three sector cells. However, it is not always necessary for three sector cells to exist and only one sector cell needs to be used to cover, for example, a road or a highway.
Returning to FIG. 1, each of the base stations B1-B10 is connected by voice and data links to a mobile switching center (MSC) 20 which is, in turn, connected to a central office (not shown) in the public switching telephone network (PSTN), or a similar facility, e.g., an integrated system digital network (ISDN). The relevant connections and transmission modes between the mobile switching center MSC 20 and the base stations B1-B10, or between the mobile switching center MSC 20 and the PSTN or ISDN, are well known to those of ordinary skill in the art and may include twisted wire pairs, coaxial cables, fiber optic cables or microwave radio channels operating in either analog or digital mode. Further, the voice and data links may either be provided by the operator or leased from a telephone company (telco).
With continuing reference to FIG. 1, a plurality of mobile stations M1-M10 may be found within the cells C1-C10. Again, while only ten mobile stations are shown in FIG. 1, the actual number of mobile stations may be much larger in practice and will generally exceed the number of base stations. Moreover, while none of the mobile stations M1-M10 may be found in some of the cells C1-C10, the presence or absence of the mobile stations M1-M10 in any particular one of the cells C1-C10 depends on the individual desires of each of the mobile subscribers who may travel from one location in a cell to another or from one cell to an adjacent or neighboring cell.
Each of the mobile stations M1-M10 includes a transmitter, a receiver, a controller and a user interface, e.g., a telephone handset, as is well known in the art. Each of the mobile stations M1-M10 is assigned a mobile identification number (MIN) which, in the United States, is a digital representation of the telephone directory number of the mobile subscriber. The MIN defines the subscription of the mobile subscriber on the radio path and is sent from the mobile station to the MSC 20 at call origination and from the MSC 20 to the mobile station at call termination. Each of the mobile stations M1-M10 is also identified by an electronic serial number (ESN) which is a factory-set, "unchangeable" number designed to protect against the unauthorized use of the mobile station. At call origination, for example, the mobile station will send the ESN to the MSC 20. The MSC 20 will compare the received ESN to a "blacklist" of the ESNs of mobile stations which have been reported to be stolen. If a match is found, the stolen mobile station will be denied access.
Each of the cells C1-C10 is allocated a subset of the radio frequency (RF) channels assigned to the entire cellular system by the concerned government authority, e.g., the Federal Communications Commission (FCC) in the United States. Each subset of RF channels is divided into several voice or speech channels which are used to carry voice conversations, and at least one paging/access or control channel which is used to carry supervisory data messages, between each of the base stations B1-B10 and the mobile stations M1-M10 in its coverage area. Each RF channel comprises a duplex channel (bidirectional radio transmission path) between the base station and the mobile station. The RF channel consists of a pair of separate frequencies, one for transmission by the base station (reception by the mobile station) and one for transmission by the mobile station (reception by the base station). Each channel unit in the base stations B1-B10 normally operates on a preselected one of the radio channels allocated to the corresponding cell, i.e., the transmitter (TX) and receiver (RX) of the channel unit are tuned to a pair of transmit and receive frequencies, respectively, which is not changed. The transceiver (TX/RX) of each mobile station M1-M10, however, may tune to any of the radio channels specified in the system.
Depending on capacity needs, one cell may have 15 voice channels, while another may have over a 100 voice channels, and corresponding channel units. Generally speaking, however, there is only one control channel (CC) in each omnidirectional or sector cell served by a base station, i.e., a base station serving an omnidirectional cell (FIG. 1) will have one control channel unit while a base station serving three sectors cells (FIG. 2) will have three control channel units. The RF (control and voice) channels allocated to any given cell may be reallocated to a distant cell in accordance with a frequency reuse pattern as is well known in the art. To avoid radio interference, all radio channels in the same cell will operate on different frequencies and, furthermore, the radio channels in any one cell will operate on a set of frequencies which is different from that used in any neighboring cell.
When in the idle state (turned on but not in use), each of the mobile stations M1-M10 tunes to and then continuously monitors the strongest control channel (generally, the control channel of the cell in which the mobile station is located at that moment) and may receive or initiate a telephone call through the corresponding one of the base stations B1-B10 which is connected to the mobile switching center MSC 20. When moving between cells while in the idle state, the mobile station will eventually "lose" radio connection on the control channel of the "old" cell and tune to the control channel of the "new" cell. The initial tuning to, and the change of, control channel are both accomplished automatically by scanning all the control channels in operation in the cellular system to find the "best" control channel (in the United States, there are 21 "dedicated" control channels in each AMPS system, i.e., their TX/RX frequencies are predefined and cannot be changed, which means that the mobile station has to scan a maximum number of 21 channels). When a control channel with good reception quality is found, the mobile station remains tuned to this channel until the quality deteriorates again. In this manner, all mobile stations are always "in touch" with the system.
While in the idle (standby) state, each of the mobile stations M1-M10 continuously determines whether a page message addressed to it has been received over the control channel. When, for example, an ordinary (landline) subscriber calls one of the mobile subscribers, the call is directed from the PSTN to the MSC 20 where the dialed number is analyzed. If the dialed number is validated, the MSC 20 requests some or all of the base stations B1-B10 to page the called mobile station throughout their corresponding cells C1-C10. Each of the base stations B1-B10 which receive the request from the MSC 20 will then transmit over the control channel of the corresponding cell a page message containing the MIN of the called mobile station. Each of the idle mobile stations M1-M10 will compare the MIN in the page message received over the control channel being monitored with the MIN stored in the mobile station. The called mobile station with the matching MIN will automatically transmit a page response over the control channel to the base station which forwards the page response to the MSC 20.
Upon receiving the page response, the MSC 20 selects an available voice channel in the cell from which the page response was received, turns the selected voice channel transceiver on, and requests the base station in that cell to order the mobile station via the control channel to tune to the selected voice channel (the MSC keeps a list of all of the channels in its service area and their status, i.e., free, busy, blocked, etc., at any time). A through-connection is established once the mobile station has tuned to the selected voice channel.
When, on the other hand, a mobile subscriber initiates a call, e.g., by dialing the telephone number of an ordinary subscriber and pressing the "send" button on the telephone handset in the mobile station, the MIN and ESN of the mobile station and the dialed number are sent over the control channel to the base station and forwarded to the MSC 20 which validates the mobile station, assigns a voice channel and establishes a through-connection for the conversation as before.
If the mobile station moves between cells while in the conversation state, the MSC will perform a "handoff" of the call from the old base station to the new base station. The MSC selects an available voice channel in the new cell and then orders the old base station to send to the mobile station on the current voice channel in the old cell a handoff message which informs the mobile station to tune to the selected voice channel in the new cell. The handoff message is sent in a "blank and burst" mode which causes a short but hardly noticeable break in the conversation. Upon receipt of the handoff message, the mobile station tunes to the new voice channel and a through-connection is established by the MSC via the new cell. The old voice channel in the old cell is marked idle in the MSC and may be used for another conversation.
In addition to call originations and page responses, an AMPS mobile station may access the cellular system for registrations. Two types of registrations are possible in AMPS: (i) periodic registration which is based on time or, more specifically, on the REGID value ("current time") and REGINCR value ("registration period") transmitted by the base station and the NXTREG value ("wake-up time") stored in the mobile station, and (ii) system area registration which is based on location or, more specifically, on the system identification (SID) transmitted in the serving cellular system. Periodic registration may be used to determine whether a mobile station is active (within radio range and switched on) or not in a cellular system. System area registration may be used to determine when a mobile station has crossed the border from one cellular system to another.
Upon receipt of a REGID message on the forward control channel (base station to mobile station), if registration is enabled in the serving cellular system, the mobile station compares the REGID value to the NXTREG value and compares the last received SID value with the value of the SID of the cellular system in which the mobile station last registered. If either the value of REGID is greater or equal to the value of NXTREG indicating that periodic registration is due, or the value of the last received SID is different than the value of the last stored SID indicating that the mobile station has travelled from one cellular system to another since the last successful registration, the mobile station will automatically send a registration access message over the reverse control channel (mobile station to base station) and will update the NXTREG value with the sum of the last received REGID value and REGINCR value, after receipt of a registration acknowledgement message on the forward control channel (the mobile station also updates the NXTREG value after each call origination or page response).
Radio Transmission Format
From its inception, the radio transmission format in cellular systems has been analog frequency modulation (FM). In each cell, the voice (analog) signals and data (digital) signals form the input signals to a transmitter (in the base station or the mobile station) which generates a sinusoidal carrier wave having a constant frequency corresponding to one of the frequencies allocated to the cell. With FM, the frequency of the carrier wave is modulated (varied) in proportion to the instantaneous amplitude of the input signal. The modulated carrier occupies a relatively narrow region of the spectrum about a nominal center frequency (the unmodulated carrier frequency). The resulting deviation of the modulated carrier wave frequency about the unmodulated (center) frequency is normally limited (by the use of bandpass filters) within a certain bandwidth, e.g., 30 KHz in the U.S., to avoid overlapping adjacent RF channels and causing adjacent channel interference. Each analog voice signal, therefore, occupies 30 KHz of spectrum, and a voice conversation requires 60 KHz.
In the conventional AMPS system, therefore, an analog speech signal modulates the carrier wave used for transmission over the RF channel. The AMPS system uses analog frequency modulation (FM) and is a single-channel-per-carrier (SCPC) system, i.e., one voice circuit (telephone conversation) per RF channel. The radio channel access scheme in the AMPS system is frequency division multiple access (FDMA) in which multiple users have access to the same set of RF channels, each user is assigned one of the available RF channels on demand, and different users are assigned different RF channels.
The Migration from Analog to Digital
Recent developments have ushered in a new digital era for cellular communications. The main driving force behind the switch to digital has been the desire to increase spectrum efficiency to meet the ever-increasing demands on system capacity. As each cellular system is allocated a finite amount of radio spectrum, capacity may be increased by reducing the amount of bandwidth required for each voice channel or, conversely, by sharing each RF channel among several voice conversations. This is made possible with the use of digital technology.
By encoding (digitizing and compressing) speech from several voice circuits prior to modulation and transmission, a single RF voice channel may be shared by several digital speech channels instead of being occupied by only one analog speech channel (one voice conversation). In this manner, the channel capacity and, consequently, the overall system capacity, may be increased dramatically without increasing the bandwidth of the voice channel. As a corollary, the cellular radio system is able to serve a substantially greater number of mobile stations at a significantly lower cost, e.g., a smaller number of channel units (transceivers) required in the base stations. Furthermore, the digital format facilitates integration of the cellular system with the emerging digital network.
In the United States, the migration from analog to digital has been spearheaded by the Electronics Industries Association (EIA) and the Telecommunication Industry Association (TIA). The EIA/TIA have undertaken the task of formulating a common air interface standard to meet industry requirements for the next generation digital cellular systems. To date, the EIA/TIA has published two separate air interface standards which are based on different radio channel multiple access schemes. The first EIA/TIA interim standard (IS) is based on a time division multiple access (TDMA) scheme and is known as the "Dual-Mode Mobile Station-Base Station Compatibility Standard" (IS-54B). The second standard is based on a code division multiple access scheme (CDMA) and is known as "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System" (PN-3118 to be published as IS-95). These standards are incorporated by reference herein (copies of the various revisions of IS-54B and PN-3118 may be obtained from the Electronics Industries Association; 2001 Pennsylvania Avenue, N.W.; Washington, D.C. 20006).
The term "dual-mode" in these two standards refers to the capability of the system to operate in either an analog or a digital mode. The analog mode of operation uses analog FM and draws on the older EIA/TIA-553 standard which is based on the AMPS standard. The digital mode of operation uses TDMA (IS-54B) or CDMA (PN-3118). The dual-mode capability facilitates the deployment of digital systems through a gradual reduction in analog capacity, i.e., the removal of RF channels from analog FM service to provide digital service. This was deemed desirable to ease the transition from analog to digital and to provide so-called "backward" compatibility with the existing analog system. Although the analog and digital modes of operation can exist alone, the goal is for them to coexist, at least in the short term, in order to allow roaming in existing systems which have not deployed the new digital technology. In the transition phase, existing analog-only mobile stations will continue to be served while the use of digital-capable mobile stations and base stations becomes more widespread.
A mobile station which complies with the defined specifications (IS-54B or PN-3118) can obtain service from an analog-only base station, a digital-only base station or an analog-digital (dual-mode) base station. The type of system serving the mobile station will depend on the availability of digital service (TDMA or CDMA) in the geographic area of the mobile station and the preference of the mobile subscriber. At call set-up or handoff, a dual-mode mobile stations can access either an analog voice channel (AVC) or, alternatively, a digital traffic channel (DTC). In analog-only or a digital-only mobile station, however, can only be assigned an AVC or DTC, respectively.
TDMA Systems
TDMA is a multiple access scheme which is based on time division multiplexing (TDM) techniques long used in the land-line telephone network to carry multiple telephone conversations simultaneously over one physical channel. In the wire-line telephone network, analog speech signals transmitted by local telephone subscribers over separate lines (subscriber loops) to the local telephone company (telco) central office are sequentially sampled and the amplitudes of the samples are quantized and then encoded into binary numbers represented by constant amplitude pulses in a process called pulse code modulation (PCM). A predetermined number of PCM channels (digital speech channels) are transmitted in a series of frames, each containing a burst of information (coded samples) from each of the PCM channels. The bursts from the different PCM channels occupy different time slots (time intervals) in each frame transmitted on the physical channel, e.g., copper wire plant. Most long-distance telephone calls are transmitted through the switching hierarchy using TDM. This technique can also be applied to transmissions on the RF channels of a cellular radio system.
An RF channel operating in TDM mode is divided into a series of repeating time slots (periodic train of time intervals) each containing a burst of information from a different data source, e.g., encoded speech from a source coder for a voice circuit. The time slots are grouped into frames of a predetermined duration. The number of time slots per frame varies depending on the number digital channels sought to be accommodated on the RF channel given the coding rates of the digital channels, the modulation level and the bandwidth of the RF channel. Each slot in a frame normally represents a different digital channel. The length of each TDM frame on the RF channel, therefore, is the minimum amount of time between two repeating time slots which are used by the same digital channel (assigned to the same user). In other words, each TDM frame consists of no more than one slot for each user.
According to IS-54B, each digital TDM RF channel can carry from three to six digital speech channels (three to six telephone conversations) depending on the source rate of the speech coder used for each digital speech channel (the modulation level and channel bandwidth are set in IS-54B). The speech coder for each digital traffic channel (DTC) can operate at either full-rate or half-rate (full-rate speech coders are expected to be used in the near future until half-rate coders are developed which produce acceptable speech quality). A full-rate DTC requires twice as many time slots in a given time period as a half-rate DTC. In IS-54B, each TDM RF channel can carry up to three full-rate DTCs or six half-rate DTCs.
The TDM RF channel frame structure for IS-54B is shown in FIG. 3. The TDM channel occupies one of the 30 KHz channels of the existing analog system. Each "frame" on the TDM RF channel comprises six equally sized time slots (1-6) and the length of the frame is 40 ms (25 frames per second). Each full-rate DTC uses two equally spaced slots of the frame shown in FIG. 3, i.e., slots 1 & 4, or slots 2 & 5, or slots 3 & 6. When operating at full-rate, the TDM RF channel may be assigned to three users (A-C), i.e., user A is assigned to slots 1 & 4; user B is assigned to slots 2 & 5; and user C is assigned to slots 3 & 6 of the "frame" shown in FIG. 3 (for full-rate, therefore, each TDM frame actually consists of three slots and not six slots, and is 20 ms long and not 40 ms long). Each half-rate DTC uses one time slot of the frame shown in FIG. 3. At half-rate, the TDM RF channel may be assigned to six users (A-F) with each of the users A-F being assigned to one of the six slots of the frame shown in FIG. 3 (for half-rate, each TDM frame actually consists of six slots and coincides with the definition of "frame" in IS-54B).
Hence, unlike an analog FDMA cellular system in which the base station and the mobile station transmit and receive continuously over an RF channel, a TDMA cellular system operates in a buffer and burst discontinuous transmission mode. Each mobile station transmits (and receives) in an assigned slot on the RF channel. At full rate, for example, the mobile station of user A would transmit on slot 1, hold for slot 2, receive in slot 3, transmit in slot 4, hold for slot 5, and receive in slot 6, and then repeat the cycle (the transmit and receive slots are offset from each other to avoid using duplexer circuitry which would otherwise be needed to allow the transmitter and receiver at the mobile station to operate simultaneously). The mobile station, therefore, transmits (or receives) in a fraction of the time (one third for full rate and one sixth for half-rate) and can be switched off to save power the rest of the time.
CDMA Systems
CDMA is a multiple access scheme which is based on spread spectrum communication techniques long used in military communications to counteract radio jamming and to protect against interception. Unlike FDMA and TDMA systems in which each transmission (signal), at any given time, is confined to its own separate frequency and its own distinct channel isolated from adjacent channels, CDMA systems transmit multiple signals simultaneously over the same spectrum band. The two chief spread spectrum techniques are frequency-hopping spread spectrum and direct-sequence or noise-modulated spread spectrum.
In frequency-hopping spread spectrum, a relatively wide band of frequencies (e.g., several MHz) is divided into a large number of much narrower channels. The transmitter "hops" from one channel to another, i.e., transmits a very short burst in one channel after another. The hopping sequence is pseudo-random, generated according to a key which is available to both transmitter and receiver. The total transmission, viewed over a longer period than the individual bursts, appears to occupy the entire bandwidth thus "spreading" the spectrum, although at any moment, for any one burst, it occupies only a small percentage of the channel. Many users can share the same channel with each user's transmissions following an orthogonal pseudorandom sequence of frequency hops.
The PN-3118 standard uses direct sequence or direct coding spread spectrum which is a digital version of noise modulation. In noise modulation, the original signal is added to (mixed with) a stronger noiselike signal with known characteristics. The resulting signal modulates a carrier for transmission to a receiver. At the receiver, a copy of the noiselike input to the transmitter is subtracted from the received signal to recover the original signal. In direct sequence, a fast rate pseudorandom binary sequence is used for the noiselike signal. This pseudonoise (PN) sequence is added to the digital information signal (e.g., digital speech) and the resulting bit stream is transmitted. At the receiver, the PN sequence is subtracted to yield the information signal. Because the transmitted signal has a high bit rate (e.g., 100 Mbps), a "spread spectrum" (i.e., wide band) is required (e.g., 100 MHz), as with frequency hopping spread spectrum. Unlike frequency-hopping spread spectrum, however, direct sequence spread spectrum transmissions occupy the entire channel bandwidth all of the time. Here also, many users can share the same channel with each user being assigned a code for generating the Orthogonal random sequence which is mixed with the information signal. The signals are separated in each receiver by using a correlator or a matched filter which accepts only signal energy from the assigned binary sequence for despreading.
FIG. 4 shows the overall structure of the forward (base station to mobile station) CDMA channel specified in PN-3118. The forward CDMA channel occupies a 1.23 MHz segment of spectrum centered on one of the 30 KHz channels of the existing analog system. According to PN-3118, the forward CDMA channel consists of up to 64 code channels (W0-W63) assigned to different uses, for example, a pilot channel (W0), a synchronization channel (W32), seven paging channels (W1-W7), and fifty five traffic channels (W8-W31 and W33-W63). Each of these code channels is spread by an orthogonal PN sequence at a fixed chip rate of 1.23 Mcps (a "PN chip" is one bit in the PN sequence). Multiple forward CDMA channels may be transmitted by a base station in a frequency division multiplexed manner.
The pilot channel carries an unmodulated direct sequence spread spectrum signal which is continuously transmitted on each active forward CDMA channel of a base station. A mobile station operating within the coverage area of the base station uses this signal for synchronization (acquisition, timing and phase reference for coherent demodulation) and for signal strength comparisons between base stations to determine when to handoff. Each base station uses a time offset of the pilot PN sequence to identify the forward CDMA channels. Different base stations are, therefore, identified by different pilot PN sequence offsets.
The sync channel is used by the mobile station to obtain system configuration and timing information (e.g., system identification, system time, pilot PN sequence offset, paging channel data rate, etc.). Each traffic channel (user) is identified by a distinct long code sequence (1.23 Mcps) which is added to the information bits prior to spreading by the PN sequence. Each paging channel is divided into a number of 80 ms time slots. A mobile station can operate in either "slotted" or "non-slotted" mode for purposes of receiving paging and control messages on the paging channel. In the slotted mode, the mobile station monitors the paging channel only during certain assigned slots. In the non-slotted mode, the mobile station monitors all slots of the paging channel.
Hybrid Systems
Some systems use a combination of access methods. The IS-54B digital cellular standard, for example, uses a combination of FDMA and TDMA. More specifically, IS-54B uses 30 Khz FDMA channels which are subdivided into 3 or 6 time slots for TDMA transmissions (3 or 6 voice calls per 30 Khz of bandwidth). Similarly, the CDMA system can also be a hybrid of FDMA and CDMA techniques where the total system bandwidth is divided into a set of wideband channels, each of which contains a large number of CDMA signals.
Personal Communications Services (PCS)
Cellular telephony had its origin in the provision of car telephone service. More recently, however, there has been an increasing shift towards the use of lightweight pocket telephones in homes, offices, public meeting places, and in virtually any other place the user can obtain service. The next step in this evolution is the emerging concept of "personal communication services" (PCS), or what has sometimes been referred to as services at "walking speeds." The idea is that not only telephone calls but also facsimile, computer data, paging messages and even video signals can be transmitted and received by a user moving around, for example, inside a building, a factory, a warehouse, a shopping mall, a convention center, an airport, or an open area.
PCS systems operate on lower power, and use smaller cellular structures than conventional wide area (vehicular) cellular systems, to provide the high-quality, high-capacity radio coverage needed for business and other applications. By reducing the transmit power of the base station, the size of the cell (or cell radius) and, with it, the frequency reuse distance are reduced resulting in more channels per geographic area. Additional benefits of a smaller cell include a longer talk-time (battery life time) for the user since the mobile station will use substantially lower transmit power than in a larger cell.
The industry has grown accustomed to using the terms "macrocell," "micocell," and "picocell" to distinguish the relative size of the cells required for a particular application (indoor or outdoor). The term "macrocell" generally refers to a cell which is comparable in size to cells in a conventional cellular telephone system (e.g., a radius of 1 Km or more). A macrocell serves rapidly moving users and covers low to medium usage areas. The terms "microcell" and "picocell," on the other hand, refer to the progressively smaller cells which are used in a PCS system, for example. A microcell serves the slowly moving users and may cover a public indoor or outdoor area, e.g., a convention center or a busy street. A picocell may cover an office corridor or a floor of a high rise building. Microcells and picocells can also cover high-density pedestrian areas or busy thorough-fares (streets or highways) in a conventional cellular system.
It is now clear that future cellular systems will likely implement a hierarchial cell structure of macrocells, microcells and picocells. From a system (MSC) perspective, the base stations in the microcells and picocells can be viewed as extensions of the base stations in adjoining or overlapping macrocells. In this case, the microcell and picocell base stations may be connected to the macrocell base station via digital transmission lines, for example. Alternatively, the microcells and picocells may be treated just like macrocells and be connected directly to the MSC.
From a radio coverage perspective, the macrocells, microcells and picocells may be distinct from each other or, alternatively, overlaid one on top of the other to handle different traffic patterns or radio environments. For example, handoff between microcells may sometimes be difficult to perform around street corners, particularly where the users are moving so rapidly that the signal strength variations are in excess of 20 dB per second. In this situation, it may be possible to use an "umbrella" macrocell for the rapidly moving users and to use microcells for the slowly moving users. By managing different types of users differently in this way, handoff between microcells may be avoided for the rapidly moving users which are subject to the severe street corner effects.
It will be readily appreciated that the capacity improvements sought for the next generation cellular systems can be achieved by more advanced macrocellular technology, e.g., digital TDMA or CDMA, or by the introduction of microcells and picocells to the specific areas where increased capacity is needed, or by a combination of both approaches. Thus, for example, analog microcells may be implemented to cover "dead spots" (areas where topography, zoning or other restrictions prevent penetration of radio signals) or "hot spots" (areas with heavy localized traffic). In this instance, coverage or capacity may be improved for the existing Subscriber base of analog mobile stations. The effectiveness of the microcellular concept in increasing capacity, however, is maximized by the use of digital technology which requires new digital-capable mobile stations.
Control Channel
The continued need to serve existing analog-only mobile stations has led to the specification in IS-54B and PN-3118 of an analog control channel (ACC) which has been inherited from the prior AMPS or the equivalent EIA/TIA-553 standard. According to EIA/TIA-553, the analog forward control channel (FOCC) on the down-link from the base station to the mobile stations carries a continuous data stream of messages (words) in the format shown in FIG. 4. Several different types (functional classes) of messages may be transmitted on the analog FOCC. These messages include a system parameter overhead message (SPOM), a global action overhead message (GAOM), a registration identification message (REGID), a mobile station control message, e.g., a paging message, and a control-filler message. The SPOM, GOAM and REGID are overhead messages which are intended for use by all mobile stations in the coverage area of the base station. Overhead messages are sent in a group called an overhead message train (OMT). The first message of each OMT must always be the SPOM which is transmitted every 0.8.+-.0.3 seconds.
The format of the analog FOCC shown in FIG. 4 requires an idle mobile station listening to the FOCC to read all the messages transmitted in each OMT (not just paging messages) even though the information contained in these messages may not have changed from one OMT to the next OMT. This requirement tends to unnecessarily limit the mobile station battery life. One of the goals of the next generation digital cellular systems, however, is to extend the "talk time" for the user, that is, the battery life of the mobile station. To this end, the co-pending U.S. patent application Ser. No. 07/956,640 (incorporated herein by reference) discloses a digital FOCC which can carry the types of messages specified for the analog FOCC, but in a format which allows an idle mobile station to read overhead messages when locking onto the FOCC and thereafter only when the information has changed, and to enter "sleep mode" at all other times. While in sleep mode, the mobile station turns off most internal circuitry and saves battery power.
The above-referenced, co-pending U.S. patent application Ser. No. 07/956,640 shows how a digital control channel (DCC) may be defined alongside the digital traffic channels (DTC) specified in IS-54B. Referring to FIG. 3, a half-rate DCC would occupy 1 slot, while a full-rate DCC would occupy 2 slots, out of the 6 slots in each 40 ms frame. For additional DCC capacity, additional half-rate or full-rate DCCs may be defined in place of the DTCs until there are no more available slots on the carrier (DCCs may then be defined on another carrier if needed). Each IS-54B RF channel, therefore, can carry DTCs only, DCCs only, or a mixture of both DTCs and DCCs. Within the IS-54B framework, each RF channel can have up to 3 full-rate DTCs/DCCs, or 6 half-rate DTCs/DCCs, or any combination in-between, for example, one full-rate and four half-rate DTCs/DCCs.
In general, however, the transmission rate of the DCC need not coincide with the half-rate and full-rate specified in IS-54B, and the length of the DCC slots may not be uniform and may not coincide with the length of the DTC slots. FIG. 6 shows the more general case of a forward DCC configured as a series of time slots. These DCC slots may be defined on an IS-54B RF channel and may consist, for example, of every nth slot in the TDM stream. In this case, the length of each DCC slot may or may not be equal to 6.66 ms, which is the length of a DTC slot according to IS-54B (there are 6 DTC slots in each 40 ms frame). Alternatively (and without limitation on other possible alternatives), these DCC slots may be defined on the paging channel specified in PN-3118 but may or may not be 80 ms long, which is the length of each paging channel slot according to PN-3118.
The DCC slots shown in FIG. 6 may be organized into higher-level structures called "superframes." Each superframe consists of logical channels which carry different kinds of information. One or more DCC slots may be allocated for each logical channel in the superframe. FIG. 6 shows an exemplary superframe which includes at least three logical channels, namely, a broadcast control channel (BCCH), a paging channel (PCH), and an access response channel (ARCH). The BCCH, which in this example is allocated 6 DCC slots, carries overhead messages. The PCH, which is allocated one DCC slot, carries paging messages. The ARCH, which is also allocated one DCC slot, carries channel assignment and other messages. The exemplary superframe of FIG. 6 may contain other logical channels, including additional paging channels. If more than one PCH is defined, different groups of mobile stations identified by different traits (e.g., last digit of MIN) may be assigned to different PCHs.
For purposes of efficient sleep mode operation and fast cell selection, the BCCH may be divided into a number of subchannels. The co-pending U.S. patent application Ser. No. 07/956,640 discloses a BCCH structure which allows the mobile station to read only a minimum amount of information at power up (when locking onto a DCC) before being able to access the system (place or receive a call). After power up, the idle mobile station needs to regularly monitor only its assigned PCH (paging slot) in each superframe and can return to sleep mode during other slots.